1. Field of the Invention
The invention concerns a measuring cable travel sensor and more particularly a measuring cable travel sensor having a braking device.
2. Discussion of the Background
Measuring cable travel sensors occur in many different configurations for example for determining the precise position of a given component which in particular can move over considerable travel distances, for example the cabin of an elevator. A typical measuring cable travel sensor of that kind and for that purpose has a pull element such as a measuring cable which is wound on a cable drum which is biassed in the cable winding-on direction. The free end of the measuring cable is connected to that component whose position is to be determined, for example as mentioned above an elevator cabin. The prestressing force for urging the cable drum in the winding-on direction is produced for example by way of a flat spiral spring which for example is arranged coaxially with respect to the cable drum and is non-rotatably connected thereto.
The cable drum is also typically coupled to a detection unit for recording the revolutions or angular distances through which the cable drum passes in the winding-on or unwinding direction, and which in addition by way of a suitable electronic evaluation system determines the length of the pull element or measuring cable which has been drawn off the cable drum.
In order to simplify that operation of determining the drawn-off length of the measuring cable and to ensure an accurate measurement result, the measuring cable is wound in only a single layer on the periphery of the cable drum, with the turns of the wound cable on the cable drum being disposed in axially juxtaposed relationship. As a result, one revolution of the cable drum will always correspond to exactly the same length of the pull element or measuring cable.
In order to ensure that the measuring cable is wound on to the drum in only a single layer and in order to prevent the measuring cable from jumping over on to the first layer to start to form a second layer there over when winding the measuring cable on to the cable drum, it is generally necessary to take suitable mechanical steps involving appropriate structure on the travel sensor to obviate this occurrence.
In that respect there is an additional difficulty insofar as such measuring cable travel sensors often have to be used in an adverse environment, for example involving a high level of fouling and contamination. For that reason the measuring cable travel sensor has to be disposed in a housing which affords sealing integrity, and the measuring cable which is passed out of the housing also has to be taken out of the housing by way of a cable guide passing through the wall of the housing, which also offers the greatest possible level of sealing integrity in relation to the cable.
In order to ensure that the measuring cable is wound on the cable drum in a neat and tidy fashion in a single layer, one possible way of achieving that is for the cable drum to be very short in its axial direction. If in addition the cable entrance is at a sufficient distance from the point on the cable drum at which the cable runs tangentially on to the cable drum, then the measuring cable, on moving from the cable entrance to the cable drum, runs only at such a slight degree of angular deflection as to still be acceptable, and such an arrangement automatically causes the cable drum which is biassed in the winding-on direction to have the measuring cable wrap there around in only a single layer.
However, measuring cable travel sensors of that kind suffer from two different difficulties which however are interrelated in terms of their origin, more specifically as follows:
In order to ensure measurement accuracy, the measuring cable has to be prestressed or biassed in the winding-on direction with a predetermined minimum force in any condition in terms of being wound on to or unwound from the cable drum and accordingly a suitably strong spring biassing effect is required to bias the cable drum in the winding-on direction.
The relatively strong spring biassing force applied to the cable drum serves inter alia to ensure that, even in the event of very fast movement on the part of the free end of the cable and the component to be measured, which is connected thereto, in a direction towards the cable drum, the measuring cable is wound on to the cable drum in a neat and tidy fashion by virtue of a sufficiently high biassing effect in the cable winding-on direction, independently of the speed with which the measuring cable is actually wound on to the drum.
Nonetheless, when the cable is wound on to the drum at very high speed, it can happen under some circumstances that the measuring cable can climb up or fly out, that is to say adopt a position at a spacing relative to the external peripheral surface of the cable winding drum, and in that case for example the measuring cable will jump over the first layer of turns already on the cable drum to form a second layer there over, being a phenomenon which is undesirable as it grossly falsifies the measurement result.
Attempts have already been made to prevent the measuring cable from climbing up over the previous turns already wound on the cable drum, for example by mechanical guide and tracking devices, distribution devices and so forth, but that tends to result in undesirable side effects such as wear, an increase in the winding force and the like, because of the mechanical friction involved.
Another detrimental effect of the high level of cable drum biassing is that, in the situation where the free end of the cable moves towards the cable drum more quickly than the cable drum can wind the measuring cable on to it or in the situation where the free end of the cable becomes detached from the component whose travel is to be measured so that the cable moves towards the cable entrance and into the travel sensor housing completely without any retardation force being applied to the cable, as a result the spring-biassed cable drum speeds up to very high speeds of rotation and at the end of the winding-on movement the free end of the cable which is usually of greater thickness and which is equipped with a fixing device for example using an eye strikes at very high speed against the cable entrance structure which is of smaller size in relation thereto. That can result in damage or even tearing of the measuring cable or can disrupt the connection between the measuring cable and its fixing device on the free end of the cable, and so forth.
An object of the present invention is to provide a measuring cable travel sensor in which, in spite of a sufficiently strong biassing effect on the cable drum in the cable winding-on direction, excessive acceleration of the cable drum in a winding-on procedure can be reliably prevented.
Another object of the present invention is to provide a measuring cable travel sensor of such a structure as to prevent the cable from climbing up on to cable turns already on the drum in a winding-on movement of the drum.
Still another object of the present invention is to provide a measuring cable travel sensor which while being of a simple structural configuration can provide for satisfactory winding-on and unwinding of the cable under properly controlled conditions, thereby to contribute to an enhanced service life for the measuring cable travel sensor.
In accordance with the principles of the present invention the foregoing and other objects of the invention are attained in a first aspect by a measuring cable travel sensor including a measuring cable and a cable drum having a winding cylinder with an external winding surface, for the measuring cable to be wound on to the external winding surface of the cable drum. The cable drum is mounted in a housing. The travel sensor further includes at least one retardation or braking magnet which is so arranged at a point that is non-rotatable with the cable drum that it has a magnetic retardation or braking action in contact-less manner in opposite relationship to the direction of winding-on rotation of the cable drum on the cable drum at an eccentric region of action thereon. At least in the region of action the cable drum includes electrically conductive material.
In accordance with the principles of the present invention the foregoing and other objects of the invention are attained in a second aspect by a measuring cable travel sensor including a measuring cable and a cable drum having a winding cylinder with an external winding surface, for the measuring cable to be wound on to the external winding surface of the cable drum. The cable drum is mounted in a housing. At least one retardation or braking magnet is eccentrically arranged on the cable drum in such a way that it has a retardation or braking effect magnetically in contact-less manner on the housing in the direction of rotation. The housing is of electrically conductive material at least in the region of action of the magnet.
In accordance with the principles of the present invention the foregoing and other objects of the:invention are attained in a third aspect by a measuring cable travel sensor including a measuring cable and a cable drum having a winding cylinder with an external winding surface, for the measuring cable to be wound on to the external winding surface of the cable drum. The cable drum is mounted in a housing. The sensor further includes at least one holding magnet arranged at the cable drum radially within the external peripheral surface of the winding cylinder of the cable drum, for holding the measuring cable radially inwardly against the winding surface, wherein the measuring cable includes magnetisable material.
In accordance with the principles of the present invention the foregoing and other objects of the invention are attained in a fourth aspect by a measuring cable travel sensor including a measuring cable and a cable drum having a winding cylinder with an external winding surface, for the measuring cable to be wound on to the external winding surface of the cable drum. The cable drum is mounted in a housing. The sensor further includes a sliding band, for example a band of textile material, more particularly a sliding band or band of other slidable, non-abrasive material, which is passed around at least a part of the external periphery of the external winding surface of the cable drum, which external winding surface is partially or entirely movable with the measuring cable, the band being passed around the winding surface at a small spacing and/or in dragging or frictional relationship at the external periphery of the winding with the measuring cable, while the spacing or the contact pressure of the band against the measuring cable winding is adjustable.
As will become apparent from the description hereinafter of preferred embodiments of the invention, it is possible to limit the speed of rotary movement of the cable drum when the measuring cable is winding thereon to by the use of a contact-less magnetic braking assembly. The components involved in that structure are to comprise an electrically conductive material.
The braking moment is generated between an eccentrically disposed region of the cable drum, preferably a region which is in the proximity of the external periphery thereof, and a stationary point which is thus non-rotatable with respect to the cable drum, for example a part of the housing of the cable drum.
As the measuring cable is wound on to the cable drum on the radially outwardly disposed peripheral surface thereof, the magnets are preferably oriented in a longitudinal direction, that is to say parallel to the axis of rotation of the cable drum, between two components which are adjacent to each other in that axial direction.
In such an arrangement, by virtue of the rotary movement of the cable drum, and irrespective of the spacing of the retardation or braking magnet relative to the component to be influenced thereby, an eddy current is firstly produced in the component which carries the retardation or braking magnet. The consequence of that eddy current is a magnetic field which is closed by way of the portion which is not in the magnetic field, thereby producing a braking moment.
Besides the strength of the magnet used, the braking action is determined to a very great extent by the spacing between the magnet and the component to be influenced thereby, and it is for that reason that this spacing should preferably be adjustable.
The respective component which is to be subjected to the influence of the braking magnet must comprise electrically conductive material, for example aluminum. When the magnet or magnets are arranged on the cable drum, that increases the inertial mass thereof, and that therefore affords a further aspect of the invention which entails arranging the braking magnet or magnets at a stationary point for example on the housing. That would also remove the compulsion for the provision of at least a pair of braking magnets on the cable drum, as a single magnet results in a cable drum unbalance, and that arrangement also makes available more space for example for fitting a magnet holder for adjustability of the air gap.
A consideration which is in favour of arranging the magnet or magnets on the cable drum however is the possibility that the magnet or magnets can be used at the same time as a holding magnet or magnets, thus providing a functionally combined unit.
In this respect, in this specification the term holding magnet is used to denote a magnet which is intended to prevent the measuring cable from climbing up or rising away from the winding surface formed by the external surface of the winding cylinder portion of the cable drum, insofar as the material of the measuring cable, which for this purpose is necessarily magnetisable, is drawn radially inwardly by magnetic force towards that winding surface. Accordingly holding magnets of that kind have to be disposed radially within the winding surface on the cable drum, and preferably therefore fixed on the inside of the winding cylinder which consists of a thin material, preferably in turn distributed over the periphery of the cable drum. In this case also the recommendation is for the axis of magnetisation of the magnet or magnets to be oriented parallel to the axis of rotation of the cable drum, that is to say, it is recommended to provide a bar magnet, in particular a permanent magnet, which extends in the longitudinal direction of the cable drum.
In both cases the magnetic force can be increased by the provision of pole shoes or pole pieces, that is to say involving close contact of iron materials against at least one outside of the magnet, in order thereby to reduce the level of magnetic losses. Preferably, the arrangement does not involve any covering by a pole piece in that direction in which the field lines are required to pass into or out of the magnet freely. Therefore, primarily cup-shaped pole pieces are preferred, which in the case of the retardation or braking magnet are directed with their open side towards the component to be influenced by the magnetic effect and which, in the case of the holding magnet, are directed with their open side radially outwardly towards the measuring cable.
It is also possible to achieve an increase in the level of retardation or braking force by using rare earth magnets, that is to say magnets with components of samarium, cobalt, neodymium and/or boron. In particular rare earth magnets of that kind can be used to produce disk-shaped magnets whose magnetisation axis extends in parallel relationship to the thickness of the disk through the disk and/or which in that case can be magnetised differently in a sector-like configuration.
Disk-shaped magnets of that kind can be disposed in the constricted conditions in terms of space of measuring cable travel sensors, more easily than elongate bar magnets.
In particular such disk-shaped magnets can be accommodated in the end of screw threaded pins or bolts as magnet holders which, in the component carrying them, can be moved by screwing closer towards or further away from the component to be influenced by the magnetic effect, in order thereby to provide for adjustability of the desired effect.
As an alternative and/or supplemental to the holding magnet or magnets, as indicated above, it is further possible in accordance with the invention to pass around the outside periphery of the movable cable drum a sliding band or belt, in particular a textile band or belt or a felt band or belt or more particularly a belt of plastic material such as PE, POM or PTFE, which when the measuring cable is wound correctly on the cable drum does not involve any contact with the wound turns of the measuring cable or bears against them without applying any force thereto, but which in contrast applies a force to the measuring cable if it climbs up on to a previous turn already formed on the cable drum.
Further objects, features and advantages of the invention will be apparent from the description hereinafter of preferred embodiments of the invention.